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After the follower has truncated its log, it will begin fetching as it does today. It will similarly include the current leader epoch, which will be validated by the leader. If the fetch response contains either the FENCED_REPLICA or UNKNOWN_LEADER_EPOCH error code, the follower will simply retry since both errors may be transient (e.g. if the propagation of the LeaderAndIsr request is delayed).   

Note that we have implemented the this fetching model in TLA: https://github.com/hachikuji/kafka-specification/blob/master/Kip320.tla. So far, we have not found any errors checking this model.

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